This invention relates to the treatment of aqueous effluents and, more particularly, to a process for treatment of spent acid etch solution, particularly acid solutions having heavy-metal contaminants by reduction of the heavy metal zinc from such etchant to levels safe for discharge.
As regulatory rules for waste stream effluent become more stringent, a method to remove heavy metals becomes mandatory. Currently, magnesium photoengraving plate is etched in a solution containing nitric acid and an organic additive. Typically, the magnesium sheet contains by weight three percent (3%) aluminum and one percent (1%) zinc with other impurities being less than 0.05 percent (0.05%). As the magnesium sheet is processed in the etch bath, aluminum, zinc, magnesium and minor impurities are leached into the acid bath. The spent acid bath generally contains 50,000 ppm magnesium and 500 ppm zinc. While a locality may allow a discharge of 5 ppm zinc, national or other governmental regulations may establish a discharge limit which is more restrictive, e.g., less than 2 ppm.
An acceptable practice, currently in use at many locations, for removing zinc ions from the etch solution involves the addition of sodium hydroxide. The hydroxide ion neutralizes the acid and, by raising the pH above 9.2, insoluble zinc hydroxide is formed. Solids can be removed from a hydroxide treated bath by either settling and decanting the supernant (aqueous layer without solids) or by filtration. Such a hydroxide precipitation method of zinc removal is complicated by simultaneous precipitation of most of the dissolved magnesium, which is not a target metal, thereby adding significantly and undesirably to the volume of solids of which are to be disposed.